Happy now?

The first London Marathon in 1981 attracted 6,500 runners. This Sunday, 35,000 will tackle the 26.2 mile course and hundreds of thousands more will compete in the 600 marathons around the world this year. Stephen Moss on how this once elite event became the favourite challenge of our couch-potato society

The following correction was printed in the Guardian's Corrections and Clarifications column, Saturday April 17 2004

In the article below we said: "London is the sole marathon in the UK - its success has discouraged other cities, which have sponsored half-marathons and fun runs instead." There are, in fact, many marathons held in the UK each year. See www.runnersworld.co.uk (click on "events").

It is spring and these days that means one thing - marathon mania. On Sunday, a group of slender Africans running absurdly quickly, 30,000 slightly tubbier Britons and large numbers of people dressed as chickens will be pounding the streets of London. On Monday, thousands more (though fewer chickens) will take part in the 108th Boston Marathon - the daddy of them all, if we exclude Phidippides' run from Marathon to Athens on the grounds that it wasn't computer timed. The most maniacal of marathon runners will do both, hopping on a plane on Sunday evening in time to start all over again at noon in Boston. But why?

"It's something I've always wanted to have done," says 28-year-old Nicola Wilson, "a bit like having a baby but much more painful." (Note the tense: she doesn't want to do it; she wants to have done it.) "I started training in January, pounding the pavements in the rain and snow. My friends think I'm mad. This is the first time and the last time. I'm never going to do it again."

Wilson is registering for the race at the Excel exhibition centre in London's Docklands, along with thousands of other quite sane-looking people - thirtysomething City workers in suits, middle-aged men in fleeces with severe haircuts, and a surprising number of couples. A large banner proclaims the marathon's philosophy: "Impossible is nothing." Think about it. The famous records associated with the event are also displayed. Fastest time for a pantomime horse - 4 hours 37 minutes. Fastest conga - 25 hours 13 minutes. Fastest time running backwards - 3.56. Skipping - 5.46. Fastest trumpet player - a mind-blowing 3.06. Biggest fundraiser: £1.18m. Jeffrey Archer has vowed to break the fundraising record this year. And probably the trumpet-blowing record too.

Marathon running is booming. "The idea of running a marathon has captured people's imaginations," says Andy Barber, deputy editor of Running Fitness magazine. "It is no longer an elite event." The first London marathon, in 1981, attracted 6,500 runners, and just two people (or was it three?) in fancy dress: a pantomime horse and a waiter with a tray of drinks. Sunday's field will number 35,000 (the maximum that can run), several thousand in absurd costumes. The total who applied to run exceeded 100,000.

No one seems to know exactly why they will be there on Sunday. "I want to challenge myself," is the most frequent refrain. It's a goal, a target, a marker, a way of proving something to yourself. "For a lot of people, it's a rite of passage," says Steve Seaton, editor of Runner's World. It is also a reflection of our society - sedentary, bloated, anxious. While almost all the best marathon runners are from the developing world, marathon running as hobby or self-imposed horror is a first-world phenomenon, appealing to the deskbound. It used to be a sport; now it's therapy - a relatively risk-free way (there have been eight deaths in the London race) of pushing ourselves to the limit.

London is the sole marathon in the UK - its success has discouraged other cities, which have sponsored half-marathons and fun runs instead - but there are now 600 major marathons around the world. About half of them are in the US, where marathon running has raced ahead in the past 10 years. At the start of the 1990s, there were 26 US marathons with more than 1,000 finishers; by the end of the decade there were 45. The number of finishers leapt from 260,000 in 1990 to 435,000 in 1999, with the increase most marked among women, who accounted for a third of all finishers in 1999 - up from 19% at the beginning of the decade. London is experiencing a similar boom among women runners.

If the typical British marathon runner is happy to do it once, Americans tend to become trainspotters, clocking up as many as they can. The 50 & DC Marathon Group was formed for runners aiming to run a marathon in every state - a feat already achieved by more than a hundred obsessives. Irritatingly for Team USA, however, the all-comers' record is held by a German, Horst Preisler, who has run more than 1,000 marathons.

Seaton is one of a new breed of tourists who go round the world running marathons. Last week, he took part in the North Pole Marathon, which is run in snowshoes (he completed the course in 4 hours 40 minutes). He has also run the Inca Trail Marathon - a high-altitude race in the Andes. Other events that will appeal to those weary of Woolwich are the Sahara Marathon in Algeria, the Six Foot Track Ultra Marathon (46km) in Australia's Blue Mountains, the Thailand Temple Run in Bangkok, the Dead Sea Marathon in Jordan, the Edge of the World Marathon in Canada and the Giant Jumping Rat Marathon in Madagascar.

One theory is that the people who run marathons are primarily men suffering mid-life crises. Running 26.2 miles in a fast-ish time, say 3.45, is a way of reasserting your youth, reclaiming your dignity and reducing your waist size. The queues of people registering for Sunday's race belie such a simplistic view - all human life is here. But, concedes Seaton, "we see a lot of what we call second-sport males. People who played team sports in their late 20s, but then found their life got busier and their weight went up. They are attracted to running because it is flexible and fits in with their schedule. They are attracted to the marathon because they tend to be very goal-orientated. They are not necessarily having a midlife crisis, but they may well be having a midlife sporting crisis."

"A lot of people respond to the challenge of running a marathon, but they come to it from different directions," says Barber. "Some are spurred to run one because of personal tragedy, and they perhaps want to raise money in memory of the person they have lost. Others are doing it because they perceive themselves to be in decline. People who are looking for a challenge will always find it in the marathon."

The London Marathon was the inspiration of Chris Brasher, gold medal-winning steeplechaser, Observer sports editor and self-made millionaire. Brasher had latched on to the jogging boom in the US and run in the autumn marathon in New York, which began in 1970. He was instantly converted. "To believe this story," he wrote in the Observer, "you must believe that the human race can be one joyous family, working together, laughing together, achieving the impossible ... the greatest folk festival the world has seen." He was determined that London should have its own running extravaganza.

Guardian athletics writer Duncan Mackay, in his obituary of Brasher, who died last year, emphasised the way he transformed perceptions of the event. "Before Brasher came along, the marathon in Britain was the precious preserve of a highly trained elite. It was the toughest event on the Olympic programme: men died attempting to complete it. To think that the couch potatoes of the video age might be tempted to train for and run such a distance was unthinkable."

"What we've got is a cross between the Olympic Games and the Notting Hill Carnival," says David Bedford, race organiser and former 10,000m world record holder. The combination of all the best marathon runners in the world and some of the worst makes for a unique event.

Commentator Brendan Foster will care passionately whether the tall Ethiopian beats the little Japanese and the rugged Mexican; everyone else will just hope that the rugby team dressed as the Sydney Opera House collapses in a heap somewhere near the Cutty Sark. The abiding image is that sea of bobbing heads and bibulous bodies slowly emerging from Greenwich Park, ready to take on the 26.2 miles to Westminster that lie ahead. Paula Radcliffe's world record in last year's race struck a blow for the elite, but the real star of the London Marathon is an Everyman struggling, to quote Brasher again, to win "a pointless but wonderful victory over mental doubt and bodily frailty".

One of the plodders on Sunday will be Michael Ward, a 46-year-old antique dealer from Bromley, Kent. Ward is unusual in that he has done absolutely no training for the event. "I haven't even tried on a pair of shorts yet," he says. "I just haven't had any time to train. I start work at three or four in the morning, have a young family, there just aren't enough hours in the day. All I've done is to make sure I eat a fried breakfast every day."

So why bother? "It's something that I've always wanted to do," says Ward, "a bit like climbing a mountain because it's there. I'll also raise a bit of money for the local school, where I'm a parent governor." But surely training is essential? "I'm not looking to win it," he says incontrovertibly. "Training just wears your body out. I'd rather save my energy for the day."

Ward would have more trouble getting into the Boston Marathon, for which you have to pre-qualify. To run in Boston, a 46-year-old would have to be able to complete the 26 miles 385 yards in three and a half hours. No mean feat - and definitely the stuff of light pasta lunches rather than fried breakfasts. The Boston event, founded in 1897 (a year after the first modern marathon at the inaugural 1896 Olympics in Athens) remains close to the original concept of a race; the London Marathon is primarily a festival. Ward, who would be banned in Boston, will be lauded in London.

"We give as much respect to the chicken as we do to the winner," says Bedford. "Obviously the chicken doesn't get paid like the elite athletes, but as far as the system is concerned everyone gets treated the same. In what other sporting event can you line up with the world champion, the Olympic champion and the world record holder? All you can do is watch at Wembley and Wimbledon. Here you can take part."

Some running pros are a bit sniffy about marathons, with their once-in-a-lifetime participants. "The challenge of the marathon has diminished slightly in recent years," says Jason Henderson, editor of Athletics Weekly. "So many people have done it now. That's one reason why I've never done one and chose to do a triathlon instead. I thought I'd try something a bit harder."

Barber also points to the irony that as more and more overweight cabbies complete marathons, elite running in the UK is in decline. Men's running is especially dire: last year Paula Radcliffe ran faster than any British man. "The number of runners able to run a really fast marathon is falling," says Barber, who himself has a personal best of 2.31. So while the UK will hold all the records for running backwards, conga-ing and completing a marathon in a diving suit, we're becoming hopeless at actually running it in shorts and vest very, very quickly.

But Bedford, banging the drum for his universal challenge, holds out a tantalising prospect. "Maybe someone that we've never heard of will suddenly have the race of his life and win. It's a very small chance, but you never know. In life, ridiculous things happen. There's that millionth of a per cent chance that someone dressed as a chicken will run a blinder."